This invention relates to a system of multiplexing coded picture data and output data of terminal units by adaptively changing the bit sharing.
One example of the multiplexing system is disclosed in U.S. Pat. No. 3,666,888. According to this patent, sound and data are multiplexed to occupy time slots from which a horizontal synchronization code word has been removed. This system, however, is disadvantages in that it requires a unique word at the interface between the picture and data, and that the percentage of time slots occupied by the picture signal is fixed.
According to another multiplexing system, since an encoder operating at the same clock frequency as that at the transmission path is used, it is necessary for multiplexing encoded picture data and terminal unit output data to use a transmission path having a higher clock frequency f.sub.l +.alpha. than the clock frequency f.sub.l of the existing transmission path.
In an encoder operating at a clock frequency f.sub.t independent of the clock frequency f.sub.l of the transmission path, since there is no data detection circuit that detects presence or absence of the terminal unit output data, the time slot for transmitting terminal unit output data is exclusively shared for the transmission of the terminal unit output data irrespective of the presence or absence of the terminal unit output data. As a result, in the absence of the terminal unit output data, a dummy data is inserted in the time slot adapted for transmitting the terminal unit output data. For this reason, the transmission path is not efficiently utilized. Moreover, as it is impossible to increase the percentage of the time slots occupied by the coded data, it is impossible to improve the picture quality.